In optical disc technologies, data can be read out from a rotating optical disc by irradiating the disc with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disc.
On a read-only optical disc, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disc. On the other hand, on a rewritable optical disc, a recording material film, from/on which data can be read and written optically, is deposited by an evaporation process, for example, on the surface of a substrate on which tracks with spiral lands or grooves are arranged. In writing data on a rewritable optical disc, data is written there by irradiating the optical disc with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
It should be noted that the depth of the pits, the depth of the tracks and the thickness of the recording material film are smaller than the thickness of the optical disc base material. For that reason, those portions of the optical disc, where data is stored, define a two-dimensional plane, which is sometimes called a “storage plane” or an “information plane”. However, considering that such a storage plane actually has a physical dimension in the depth direction, too, the term “storage plane (information plane)” will be replaced herein by another term “information storage layer”. Every optical disc has at least one such information storage layer. Optionally, a single information storage layer may actually include a plurality of layers such as a phase-change material layer and a reflective layer.
Also, in this description, the “information storage layer” will sometimes be referred to herein as just a “layer” for the sake of simplicity. Thus, a single optical disc with N “information storage layers” that are stacked one upon the other will be referred to herein as an “N-layer optical disc”. For example, a BD with two information storage layers will be simply referred to herein as a “dual-layer (which means “two-layer”) BD”.
In high-density optical discs such as BDs that are sometimes called “next-generation DVDs”, at least one information storage layer is supported on a substrate and the light incident surface of the information storage layer is covered with a thin protective coating (which is a light-transmitting layer). If a number of information storage layers are stacked one upon the other, then another light-transmitting layer is interposed between each pair of those information storage layers. The depth of an information storage layer in question (i.e., the information storage layer at which the focus of a light beam is currently located) as measured from the light incident surface of such an optical disc (which will be simply referred to herein as “the surface of an optical disc”) is typically 100 μm or less. In this description, the depth of each information storage layer as measured from the light incident surface of an optical disc will sometimes be referred to herein as “the light-transmitting layer thickness” of the information storage layer in question.
To read data that is stored on an optical disc or to write data on a rewritable optical disc, the light beam always needs to maintain a predetermined converging state on a target track on a target information layer. For that purpose, a “focus control” and a “tracking control” are required. The “focus control” means controlling the position of an objective lens perpendicularly to the information storage layer (which direction will be referred to herein as a “substrate depth direction”) such that the focus position (or focal point) of the light beam is always located on the information storage layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of an optical disc loaded (which direction will be referred to herein as a “disc radial direction”) such that the light beam spot is always located right on a target track.
To get the focus control and tracking control done, a focus error or a tracking error needs to be detected based on the light reflected from an optical disc and the location of the light beam spot needs to be adjusted so as to minimize the error. The magnitudes of the focus error and the tracking error are respectively represented by a “focus error (FE) signal” and a “tracking error (TE) signal” that are generated based on the light reflected from the optical disc.
Once a fingerprint is left on the surface of an optical disc, the intensity of the reflected light will decrease when the light beam passes through that fingerprint. As a result, the intensity distribution of the light beam will decrease non-uniformly, thus causing a disturbance in the TE signal and producing tracking abnormality. Consequently, some type of tracking failure such as a track jump happens. Various techniques have been proposed so far to avoid such tracking failures. For example, according to Japanese Patent Application Laid-Open Publication No. 2006-179136, the decrease in the intensity of the reflected light is detected on a regular basis, and if any defect, including the presence of a fingerprint, has been detected, the user is notified of that, thereby avoiding predictable write failures. Meanwhile, if the user permits such a soiled disc, a read/write operation will be performed on it just as instructed by him or her. Furthermore, by providing means for avoiding a repetitive notification, an opportunity to perform a read/write operation just as planned can be guaranteed while avoiding write failures to be caused by the dirt on the disc.
Meanwhile, as infrastructures to implement terrestrial digital broadcasting have been rapidly set up and as TV sets with a reduced thickness and a huge screen have become more and more popular among general consumers, there are growing demands for digital high definition TV broadcasting with high image quality. Considering these circumstances, it is expected that the mainstream storage media will soon change from DVDs into BDs. Digital broadcasts of high quality are transmitted in an MPEG-TS compression format at a transfer rate of 16 to 24 Mbps, which is higher than that of conventional analog broadcasts. A broadcast with that high transfer rate could be recorded for only about 20 minutes on a DVD but for 4.5 to 6 hours on a dual-layer BD with a storage capacity of 50 GB. Recently, to further extend the maximum recording duration, multilayer BDs in which three or more information storage layers are stacked one upon the other have been researched and developed extensively. For example, it has been either made official or proposed in articles that three- to six-layer BDs (with a storage capacity of 200 GB) and 16- and 20-layer BDs (with a storage capacity of 500 GB) are under development. If such multilayer optical discs, of which the number N of information storage layers stacked is three or more, are realized, sufficiently long recording duration can be ensured by just a single BD even without building in a hard disk.